U.S. patent number 4,415,877 [Application Number 06/371,343] was granted by the patent office on 1983-11-15 for gas sensing element.
This patent grant is currently assigned to NGK Spark Plug Co., Ltd.. Invention is credited to Hideki Kudo, Tsutomu Saito, Toshifumi Sekiya, Akio Takami.
United States Patent |
4,415,877 |
Takami , et al. |
November 15, 1983 |
Gas sensing element
Abstract
A gas sensing element having an oxide semiconductor element is
provided with lead wires partially embedded therein. The lead wires
are formed of a nickel core and a cover layer of at least 5.mu. in
thickness and made of platinum or platinum alloy containing from 1
to 10 percent low catalytic metal.
Inventors: |
Takami; Akio (Nagoya,
JP), Saito; Tsutomu (Nagoya, JP), Sekiya;
Toshifumi (Nagoya, JP), Kudo; Hideki (Nagoya,
JP) |
Assignee: |
NGK Spark Plug Co., Ltd.
(Aichi, JP)
|
Family
ID: |
13221651 |
Appl.
No.: |
06/371,343 |
Filed: |
April 23, 1982 |
Foreign Application Priority Data
|
|
|
|
|
Apr 25, 1981 [JP] |
|
|
56/63175 |
|
Current U.S.
Class: |
338/34;
174/126.2; 338/322 |
Current CPC
Class: |
G01N
27/12 (20130101) |
Current International
Class: |
G01N
27/12 (20060101); H01L 007/00 () |
Field of
Search: |
;338/34,35,25,28,322,324
;174/126CP ;219/541 ;73/27R ;422/98 ;428/670,607,620 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Albritton; C. L.
Attorney, Agent or Firm: Sughrue, Mion, Zinn, Macpeak &
Seas
Claims
What is claimed is:
1. A gas sensing element, comprising:
a semiconductor formed of a sintered metal oxide and having an
electrical resistance varying with a gas component to be
sensed;
a pair of lead electrodes embedded in said semiconductor, each of
said electrodes comprising a nickel wire core and a cover layer
surrounding said core, said cover layer comprising an alloy
essentially containing platinum metal and including at least one
low catalytic metal in a range of from 1 to 10% by weight.
2. A gas sensing element as claimed in claim 1, wherein said low
catalytic metal is selected from the group consisting of gold,
nickel, cobalt, iron, silver, titanium and copper.
3. A gas sensing element as claimed in claim 1, wherein said cover
layer is a clad layer.
4. A gas sensing element as claimed in claim 3, wherein in said
clad cover layer said low catalytic metal is selected from the
group of gold, nickel, cobalt and iron.
5. A gas sensing element as claimed in claim 1, said cover layer
being a vapor evaporated layer.
6. A gas sensing element as claimed in claim 1, said cover layer
being a plated layer.
7. A gas sensing element as claimed in claim 1, said cover layer
having a thickness of at least 5.mu..
Description
BACKGROUND OF THE INVENTION
This invention relates to a gas sensing element using a
semiconductor made of sintered metal oxide, for detecting the
partial pressure of oxygen gas in an gas atmosphere from a
variation in the electrical resistance of the semiconductor.
The conventional gas sensing element includes a semiconductor made
of a sintered oxide of a metal such as titanium dioxide cobalt, tin
or chromium, and a pair of electrodes embedded in the
semiconductor, as lead wires across the electrical resistance of
the semiconductor. This gas sensing element has been extensively
employed as a gas component detecting device to control the exhaust
gas of a combustion machine, such as an internal combustion engine,
to decrease emission.
To form a gas sensing element of this type, a pair of electrodes
are embedded in a mass of metal oxide powder, which is the raw
material of the semiconductor. The electrodes and the mass of metal
oxide powder are molded in a die under pressure and are then
subjected to sintering at a temperature of 1200.degree. C. or
higher. Therefore, even if sintering is carried out in an inert
atmosphere, the electrodes are unavoidably oxidized. Accordingly,
the electrodes are made of expensive platinum or a platinum alloy
such as platinum-rhodium (hereinafter referred to as "platinum
metal"), to prevent oxidation. On the other hand, the semiconductor
made of the sintered oxide (hereinafter referred to as "an oxide
semiconductor") is made porous to increase the gas sensing ability.
Therefore, CO and HC in the exhaust gas enter the porous
semiconductor and come in contact with the platinum metal
electrodes, and carbon is deposited by their catalytic reaction.
This carbon accumulates on the electrodes, causes stress on the
oxide semiconductor, expanding the latter, and occasionally
destructive failure are occurred. In this point, the conventional
gas sensing element is disadvantageous.
SUMMARY OF THE INVENTION
An object of this invention is to eliminate the above-described
drawback accompanying a conventional gas sensing element. A feature
of the gas sensing element according to the invention resides in
that nickel wires, covered to a thickness of at least 5.mu. with an
alloy of essentially platinum metal and a low catalytic metal, are
employed as a pair of electrodes embedded in the oxide
semiconductor, for leading out the electrical resistance of the
oxide semiconductor.
BRIEF DESCRIPTION OF THE DRAWINGS
The single FIGURE in the accompanying drawing is a perspective view
of a gas sensing element according to this invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
Examples of catalytic metals which may be used in the invention are
gold, silver, nickel, cobalt, iron, titanium and copper. The nickel
wires are covered with an alloy essentially of platinum metal and
one or more of these low catalytic metals in a range of from 1 to
10% by weight. As a result, the electrodes are not oxidized when
heated together with the oxide semiconductor, and deposition of
carbon during the catalytic reaction is prevented, although the
electrodes come in contact with CO and HC gases during the
operation.
A cover layer may be formed on the nickel wire with the platinum
metal alloy according to conventional methods such as metallurgical
clad chemical plating, vacuum evaporation, sputtering, and
metallizing methods. However, in view of work efficiency,
metallurgical clad, the chemical plating method and the vacuum
evaporation methods are more suitable. It is desirable that the
cover layer be a clad layer, because it is uniform in thickness and
no pin holes are created therein. Since the cover layer must be
sufficiently high in dictility and flexibility in rolling and
withdrawing a clad wire, the low catalytic metal to be mixed with
the platinum metal is selected from a group of metals such as gold,
nickel, cobalt and iron which will create no intermetallic compound
with the platinum metal.
In the invention, the content of the low catalytic metal is limited
to 1 to 10% with respect to that of the platinum metal because if
the content of the low catalytic metal is less than 1%, the effect
is considerably low, and if it exceeds 10%, the electrodes are
broken when used for a long time.
The reason why the electrodes are broken is estimated to be as
follows:
When the electrodes are heated for a long time during operation, an
intermetallic compound is formed between the low catalytic metal
and the platinum metal, as a result of which the cover layer is
embrittled. Furthermore, the thermal expansion coefficient of the
cover layer made of the platinum metal alloy is about
9.times.10.sup.-6, while that of the nickel wire is much higher,
about 14.times.10.sup.-6. Accordingly, the cover layer thus
embrittled cannot follow the thermal expansion of the nickel wire,
and finally it is cracked, which results in the breakage of the
electrode.
The cover layer should be as thin as possible because the thickness
of the cover layer does not directly concern its durability. In the
case where the cover layer is formed by chemical plating, vacuum
evaporation, or sputtering as was described before, it should have
a thickness of at least 5.mu., because in this case no pin holes
are formed therein, which increases the acid resistance when the
cover layer is sintered together with the oxide semiconductor. The
clad cover layer should also have a thickness of at least 5.mu. in
view of the convenience of manufacture. It is desirable for both
cover layers to have a thickness of about 50.mu. in view of both
manufacturing cost and stability.
EXAMPLE
A pair of electrodes 2 were embedded in an oxide semiconductor 1 to
a depth of 1 mm, with 2.5 mm being exposed, and were spaced by 1.2
mm from each other, to fabricate a gas sensing element. The oxide
semiconductor 1 was of titanium dioxide and had a porosity of about
30%. After being sintered, the oxide semiconductor 1 was 2 mm in
thickness, 2.5 mm in width and 3.5 mm in length and had slightly
rounded corners. Each electrode 2 was prepared by forming a cover
layer 2b of 50.mu. in thickness by cladding an alloy containing
essentially platinum metal and low catalytic metal, on the surface
of a nickel wire 2a of 0.35 mm diameter.
The electrodes 2 of the gas sensing element 1 thus fabricated were
inserted into a pair of insulating tubes (not shown), each being 50
mm in length and having an inside diameter of 0.9 mm, and were
sealed with heat-resisting cement (not shown). The insulating tubes
were secured to the exhaust pipe of a four-cylinder 1800 cc. car
engine in such a manner that the gas sensing element was set at the
center of the exhaust pipe. Under this condition, an electrode
endurance test was carried out under full load. The test results
detected at time intervals of 50 hours are as indicated in the
following Table. It should be noted that a number of electrodes
were prepared using different materials, and three specimens of
each electrode were subjected to the endurance test. Therefore,
each endurance test result is represented by the shortest time
required for carbon deposition and the shortest time required for
electrode breakage in the Table.
TABLE
__________________________________________________________________________
ENDURANCE TEST COVER LAYER RESULTS COMPOSITION ELECTRODE SPECIMEN
PLATINUM LOW CATALYTIC CARBON DEPOSITION BREAKAGE TIME NO. METAL
METAL TIME (Hr) (Hr) REMARKS
__________________________________________________________________________
1 Pt -- 50 200 REFERENCE 2 87Pt--13Rh -- 100 At least 400 OUT OF 21
Pt 99.5 Au 0.5 100 At least 400 THE RANGE 22 Ni " " " 23 Co " " "
24 Fe " " " 25 Ag " " " 26 Ti " " " 27 Cu " " " 31 Pt 99.0 Au 1 250
At least 400 WITHIN 32 Ni " " " THE RANGE 33 Co " 200 " 34 Fe " " "
35 Ag " " " 36 Ti " " " 37 Cu " " " 41 Pt 95.0 Au 5.0 At least 400
At least 400 WITHIN 42 Ni " " " THE RANGE 43 Co " " " 44 87Pt--13Rh
Au " " " 45 Total:95.0 Ni " " " 46 Co " " " 47 Ti " " " 51 Pt 90.0
Au 10.0 At least 400 200 WITHIN 52 Ni " " " THE RANGE 53 Co " " "
54 Fe " " " 55 Ag " " " 56 Ti " " " 57 Cu " " " 61 Pt 85.0 Au 15.0
At least 400 100 WITHIN 62 Ni " " 150 THE RANGE 63 Co " " 100 64 Fe
" " " 65 Ag " " 50 66 Ti " " " 67 Cu " " "
__________________________________________________________________________
As is apparent from the above-described Table, the electrodes
indicated by specimen numbers 31 through 37, 41 through 47 and 51
through 57 whose cover layers were formed with platinum metals
which contained 1 to 10% low catalytic metals, Au, Ni, Co, Fe, Ag,
Ti and Cu by weight showed no abnormal results after a 200 hour
endurance test, and especially ones with Au, Ni, Co and Ti
contained therein provided significantly satisfactory results. On
the other hand, the electrodes whose cover layers were formed with
platinum metal typically platinum or platinum-rhodium, which did
not contain the above-described low catalytic metal, deposited
carbon in fifty to one hundred hours. In the case of platinum, the
electrode was broken in 200 hours; and in the case of
platinum-rhodium, the electrode was broken in 400 hours. It can be
estimated that the electrode breakage was due to the accumulation
of carbon, in both cases. The electrode indicated by specimen
numbers 21 through 27 whose cover layers were formed with alloy
containing less than 1% low catalytic metal by weight, deposited
carbon in 100 hours which might result in the semiconductor cracks.
On the other hand, in specimen numbers 61 through 66 in which the
content of low catalytic metal was more than 10%, no carbon was
deposited, but electrodes were broken in 50 to 150 hours due to
brittle properties of the intermetallic compound.
The above-described Table has been prepared by using electrodes
which were fabricated by convering the nickel wires with alloys
which contain platinum and low catalytic metal, by cladding. The
cover layers may be formed by chemical plating or vacuum
evaporation, as described above. For instance, in the case of an
electrode fabricated under the same conditions as those described
except that the cover layer, similarly as in specimen number 41,
containing 95% platinum and 5% gold, was formed by chemical
plating, no carbon was deposited in 400 hours. The endurance test
result ws the same as that of specimen number 41.
Two or more low catalytic metals may be contained in platinum
metal. In the case of an electrode which was fabricated under the
same conditions as those described above except that a cover layer
of an alloy consisting of 2.5% gold and 2.5% nickel as low
catalytic metals and 95% platinum was formed by cladding, no carbon
was deposited in 400 hours. Similarly to electrodes indicated by
specimen numbers 41 and 42, the endurance test results of the
electrode thus fabricated were much better than those of the
electrode indicated by specimen numbers 21 through 27 and 61
through 67, and those of the electrodes indicated by specimen
numbers 1 and 2 which had cover layers of platinum or
platinum-rhodium only (including no low catalytic metal); however,
the effect of employing two or more kinds of low catalytic metals
was not significant.
As was described above, the gas sensing element according to the
invention includes the semiconductor prepared by sintering a metal
oxide and one pair of electrodes which are fabricated by coating
the surface of two nickel wires with an alloy which contains
platinum metal and one or two low catalytic metals in a range of
from 1 to 10% by weight, the electrodes being embedded in the
semiconductor. Accordingly, with the gas sensing element of the
invention, the consumption of expensive platinum metal is greatly
reduced when compared with that of the conventional gas sensing
element in which the platinum metal wires are employed as
electrodes. Furthermore, as is apparent from the example described
above, the endurance has been remarkably improved according to the
invention.
* * * * *